Could someone please explain what happens when steel is heat treated and why these happenings cause the steel to become harder? Please dig down into the micro details of the crystallites but in somewhat laymen's terms. Also describe the processes which achieve these results; if you have the time. Thanks for your time and effort.
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Alright, usually whu heat you heat treat mild steel, this is because you would like it stronger than you got it. To do this you must alter the grain structure of the part or in other words, the CRYSTALLITES. Ultimately you'll end up altering the nature of the grains structure changing the tensile strength it can handle, yield strength, and even change the elongation percentage that it can tolerate under load. Heat treatment controls the rate of diffusion, and the rate of cooling within the microstructure to create these elements. Usually what they use to do this is add an element to it to make it stronger such as carbon to increase its rockwell hardness, which by the way the lower you go the harder the part can become. Heat treatment can be used in more ways than this. It can also be used to weaken the parts grain structure. This will lower the parts mechanical properties making it softer and more ductile or easier to manipulate if you have to bend it into a particular shape. Typically if you allow the part to cool after heat treatment it does go through annealing. The part will be heat treated into different stages. The first being the austenitic crystal phase which depending on how hard and strong you want it, will be at its peak. When it is cooled, it will go through a will transform to martensite which is a hard yet brittle crystalline structure. Martenised part will usually be tempered to a certain degree to improve the mechanical properties to what is needed. There is more to this and you can use the link below to read more about it.
Mild steel is a solution of carbon within iron, etc. When the steel is very hot the carbon is well distributed. As the steel is cooled iron crystals form excluding the carbon that will then collect at the boundaries (grain boundaries) between iron crystals. If cooling is rapid at any point in the cool down crystal growth will cease (affecting final grain size and the mix of carbon and iron at the grain boundaries). The ductility or hardness of the resulting steel will be determined by the grain sizes and makeup of the carbon/iron mix between grains. Ductile iron permits 'slabs' of iron to slip past each other within iron crystals and location of carbon atoms may help prevent slip resulting in hardness. Therefore, heat treatment usually consists of 'soaking' the steel at a temperature high enough to dissolve all the carbon uniformly then cooling it down slowly or rapidly to obtain optimum grain size and interstitial carbon between grains to obtain desired properties.